Microlithography (also called photolithography or simply lithography) is a technology for the fabrication of integrated circuits, liquid crystal displays and other microstructured devices. The process of microlithography, in conjunction with the process of etching, is used to pattern features in thin film stacks that have been formed on a substrate, for example a silicon wafer. At each layer of the fabrication, the wafer is first coated with a photoresist which is a material that is sensitive to radiation, such as deep ultraviolet (DUV) light. Next, the wafer with the photoresist on top is exposed to projection light through a mask in a projection exposure apparatus. The mask contains a circuit pattern to be projected onto the photoresist. After exposure the photoresist is developed to produce an image corresponding to the circuit pattern contained in the mask. Then an etch process transfers the circuit pattern into the thin film stacks on the wafer. Finally, the photoresist is removed. Repetition of this process with different masks results in a multilayered microstructured component.
A projection exposure apparatus typically includes an illumination system, a mask stage for a aligning the mask, a projection lens and a wafer alignment stage for aligning the wafer coated with the photoresist. The illumination system illuminates a field on the mask that often has the shape of an (elongated) rectangle or a ring segment.
In current projection exposure apparatus a distinction can be made between two different types of apparatus. In one type each target portion on the wafer is irradiated by exposing the entire mask pattern onto the target portion in one go; such an apparatus is commonly referred to as a wafer stepper. In the other type of apparatus, which is commonly referred to as a step-and-scan apparatus or scanner, each target portion is irradiated by progressively scanning the mask pattern under the projection light beam in a given reference direction while synchronously scanning the substrate parallel or anti-parallel to this direction. The ratio of the velocity of the wafer and the velocity of the mask is equal to the magnification of the projection lens, which is usually smaller than 1, for example 1:4.
It is to be understood that the term “mask” (or reticle) is to be interpreted broadly as a patterning device. Commonly used masks contain transmissive or reflective patterns and may be of the binary, alternating phase-shift, attenuated phase-shift or various hybrid mask type, for example. However, there are also active masks, e.g. masks realized as a programmable mirror array. An example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. More information on such mirror arrays can be gleaned, for example, from U.S. Pat. No. 5,296,891 and U.S. Pat. No. 5,523,193. Also programmable LCD arrays may be used as active masks, as is described in U.S. Pat. No. 5,229,872. For the sake of simplicity, the rest of this text may specifically relate to apparatus including a mask and a mask stage; however, the general principles discussed in such apparatus should be seen in the broader context of the patterning devices as noted above.
The angular distribution of the projection light impinging on the mask is usually adapted to the kind of pattern to be projected onto the photoresist. For example, relatively large sized features may involve a different angular distribution than small sized features. Commonly used angular distributions of projection light are referred to as conventional, annular, dipole and quadrupole illumination settings. These terms refer to the irradiance distribution in a pupil plane of the illumination system. With an annular illumination setting, for example, only an annular region is illuminated in the pupil plane, and thus there is only a small range of angles present in the angular distribution of the projection light so that all light beams impinge obliquely with similar angles onto the mask.
In illumination systems designed for wavelengths below 200 nm, lasers are typically used as light sources. The projection light bundle emitted by a laser typically has a small cross section and a low divergence, and therefore also the geometrical optical flux is small. The geometrical optical flux, which is also referred to as the Lagrange invariant, is a quantity that is, at least for certain special configurations, proportional to the product of maximum light angle and size of the illuminated field.